This application is based on Patent Application No. 2000-195271 filed Jun. 28, 2000 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to an optical multi/demultiplexer for multiplexing optical signals at high density in an optical communication or an optical switching field.
2. Description of the Related Art
Recently, wavelength multiplexing of an increasing number of channels has been studied to meet the growing demand of communication traffic. Besides, because of the limited low-loss bandwidth of optical fibers or the limited low-noise amplification bandwidth of rate-earth doped fiber amplifiers such as Erbium-doped fiber amplifiers, optical multi/demultiplexers that can handle optical signals with a narrower channel spacing are required. Arrayed waveguide gratings (AWGs) are a typical optical multi/demultiplexer to meet such a need.
An arrayed waveguide grating, which can multiplex a large number of signals at once by itself, is applicable to an optical communication system, and is actually used in currently operating optical systems. As for an optical multi/demultiplexer like the arrayed waveguide grating that handles a number of signals (no less than eight waves), unless the crosstalk per channel is very small, it will be inapplicable to an optical communication system because of the cumulative noise. Today, arrayed waveguide gratings with a 100-GHz channel spacing achieve small enough crosstalk of less than xe2x88x9240 dB, and are in mass production already. However, as for the arrayed waveguide gratings with a narrower wavelength spacing of a 10-GHz channel spacing, the volume production has not yet implemented because of a problem in that small crosstalk cannot be obtained unless the phase error at the fabrication is precisely estimated and compensated.
Apart from the arrayed waveguide gratings, there are some optical multi/demultiplexing devices utilizing components such as fiber gratings or thin film filters. However, it is difficult for them to simultaneously satisfy such requirements as channel configuration with a precise wavelength spacing, small excess loss and small loss imbalance of individual channels. Thus, there are proposed optical multi/demultiplexers that can handle only a few channels.
To implement an optical multi/demultiplexer that can handle optical signals at a narrower wavelength spacing, a method is proposed that combines two existing arrayed waveguide gratings with a Mach-Zehnder filter with an FSR (Free Spectral Range) equal to the channel spacing of the arrayed waveguide gratings.
Using this method can construct an optical multi/demultiplexer capable of apparently doubling the number of channels of the arrayed waveguide grating using the existing arrayed waveguide gratings. In addition, since both the arrayed waveguide gratings and the Mach-Zehnder filter can be composed of the same type optical waveguides, the method does not involve any problem of connection loss, and they can be easily integrated on the same substrate. Above all, the method has an advantage of being able to suppress the crosstalk with other channels except for the adjacent channels to sufficiently small value by means of the existing arrayed waveguide gratings.
However, since the Mach-Zehnder filter has a narrow rejection bandwidth, and hence adjacent channels of the optical multi/demultiplexer constructed by the method also has a narrow rejection bandwidth, utilizing the Mach-Zehnder filter with such a transmission spectrum has a problem of being unable to sufficiently improve the adjacent channel crosstalk considering the center wavelength variations of optical signals. Besides, since the optical signal of the channel passes two filters, the passband width becomes narrower. Thus, the optical multi/demultiplexer constructed by combining the Mach-Zehnder filter with the arrayed waveguide gratings has a problem of narrowing the allowed bandwidth of the optical signal wavelength.
The present invention is implemented to solve the foregoing problems. Therefore, an object of the present invention is to provide an optical multi/demultiplexer that has a wide passband and small crosstalk over all channels, for handling optical signals of multiple wavelengths with narrow channels.
To accomplish the object of the present invention, there is provided an optical multi/demultiplexer including M arrayed waveguide gratings and a waveguide type circulating filter connected in cascade, each of the arrayed waveguide gratings having a same channel spacing and a center wavelength of a channel shifted by an amount of 1/M of the channel spacing, and the waveguide type circulating filter having M periodic output transmission spectra with a repetition period identical to the channel spacing, wherein the waveguide type circulating filter comprising: two optical waveguides; (N+1) directional couplers for coupling the optical waveguides at (N+1) different locations, where N is an integer greater than one; and means for providing the optical waveguides at N locations between the directional couplers with an optical path length difference.
Here, a first directional coupler of the directional couplers may have an angular representation of a coupling coefficient of 0.25xcfx80, where the angular representation of the coupling coefficient=sinxe2x88x921{square root over ( )} (power coupling ratio), and remaining (Nxe2x88x921) optical path length differences may be given by xc2x12 mLxc2x1mxe2x80x2xcex/2, where m and mxe2x80x2 are integers including zero, xcex is center wavelength, and L is the optical path length difference of the first optical waveguide. Here, the xe2x80x9cpower coupling ratioxe2x80x9d is defined by P2/(P1+P2) when a light signal is launched into one of the input ports of a directional coupler, where P1 (mW) is optical power output from an output port connected to the input port, and P2 (mW) is optical power output from an output port of the other waveguide.
An (N+1)th directional coupler of the directional couplers may have an angular representation of a coupling coefficient of 0.25xcfx80, where the angular representation of the coupling coefficient=sinxe2x88x921{square root over ( )} (power coupling ratio), and remaining (Nxe2x88x921) optical path length differences may be given by xc2x12 mLxc2x1mxe2x80x2xcex/2, where m and mxe2x80x2 are integers including zero, xcex is center wavelength, and L is the optical path length difference of an Nth optical waveguide.
Angular representations of coupling coefficients of the first, second and third directional couplers, and the optical path length differences at first and second locations may be give by one of two sets consisting of {xcfx80/4, (xcfx80/3)xe2x88x92x, (xcfx80/12)+x, L, 2Lxc2x1xcex/2} and {xcfx80/4,(xcfx80/6)+x, (xcfx80/12)+x, L, xe2x88x922L}, where 0xe2x89xa6xxe2x89xa60.1.
Angular representations of coupling coefficients of third, second and first directional couplers, and the optical path length differences at second and first locations may be give by one of two sets consisting of {xcfx80/4, (xcfx80/3)xe2x88x92x, (xcfx80/12)+x, L, 2Lxc2x1xcex/2} and {xcfx80/4, (xcfx80/6)+x, (xcfx80/12)+x, L, xe2x88x922L}, where 0xe2x89xa6xxe2x89xa60.1.
At least one of the directional couplers may consist of a tunable coupler.
At least one of the two waveguides that provide the N optical path length differences may include a phase shifter.
Thus, the waveguide type circulating filter in accordance with the present invention comprises the two optical waveguides and the (N+1) directional couplers that couple the optical waveguides at (N+1) locations, where N is an integer greater than one. As a result, it can widen the rejection bandwidth of the adjacent channels of the optical multi/demultiplexer including the combination of the arrayed waveguide gratings and the waveguide type circulating filter. It is preferable that the waveguide type circulating filter consist of a 2xc3x972 optical signal processor capable of having transmission spectra with a square profile having a wide passband and rejection band. This enables the crosstalk among all the channels to be suppressed to a small amount even taking account of the center wavelength variations of the optical signal.
As for the transmission channels, since the waveguide type circulating filter has a wide flat-top passband and rejection band, it has an advantage of being able to prevent the reduction of the proper passband the arrayed waveguide gratings.
Thus, utilizing the optical waveguide filter with the configuration in accordance with the present invention can provide an optical multi/demultiplexer with a wide passband and small crosstalk over all the channels, even when doubling the number of channels of the existing arrayed waveguide grating.
As described above, according to the present invention, the waveguide type circulating filter utilizes the 2xc3x972 optical signal processor that comprises the two optical waveguides and the (N+1) directional couplers that couple the optical waveguides at (N+1) locations, where N is an integer greater than one, that provides the optical waveguides with the optical path length differences at N locations between the directional couplers, and that can provide transmission spectra with a square profile having a wide passband and rejection band. As a result, it can widen the rejection bandwidth of the adjacent channels of the optical multi/demultiplexer including the combination of the arrayed waveguide gratings and the waveguide type circulating filter. This enables the crosstalk among all the channels even taking account of the center wavelength variations of the optical signal.
Furthermore, as for the transmission channels, since the waveguide type circulating filter has a wide flat-top passband, it has an advantage of being able to prevent the reduction in the proper passband of the arrayed waveguide gratings.
Thus, utilizing the optical waveguide filter with the configuration in accordance with the present invention can provide an optical multi/demultiplexer with a wide passband and small crosstalk over all the channels, even when doubling the number of channels of the existing arrayed waveguide grating.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.